The acronym NFC denotes a short-distance high-frequency wireless communication technology for exchanging data between two contactless devices over a short distance, of 10 cm for example.
NFC technology is standardized in the ISO/IEC 18092, ISO/IEC 21481 and NFC Forum documents, but incorporates a range of pre-existing standards including the type A and type B protocols of the ISO/IEC 14443 standard.
An NFC device may be generally used in either reader or card mode to interact with another contactless device, for example by using a contactless communications protocol such as protocol A of the ISO/IEC 14443 standard.
In card mode, the NFC component acts as a transponder, for example a card or a label, and interacts with the external device which is a reader.
There are numerous applications, such as passing through payment barriers in transport systems (the mobile telephone acting as a travel ticket) or payment applications (the mobile telephone acting as a credit card).
The device emulated in card mode may be passive or active. A passive device carries out load modulation on the magnetic field generated by the reader. An active device uses active load modulation (also known by those skilled in the art under the acronym of ALM) to transmit information to the reader. Furthermore, the device also generates a magnetic field via its antenna which simulates the load modulation of the reader field carried out by a passive device.
Active load modulation is advantageously used when the signal resulting from passive load modulation is not strong enough to be detected by the reader. This is the case, notably, when the antenna of the device is small or located in an unfavorable environment.
When a card is detected by the reader, in a known conventional manner, the reader initiates communication by modulating the magnetic field that it generates. This is a reception phase for the card. When the reader has terminated this step, the card responds by generating its own magnetic field and modulating it during transmission frames. There are then two possible situations.
In a first situation, each frame may comprise a series of bursts (as they are known in English) of ALM carriers, separated by spaces in which the card emits no information and no electromagnetic field. The time interval between the initial instants of two consecutive bursts is equal to the period of a subcarrier used for the modulation.
In some cases, the modulation must be increased. This is done by generating, in an uninterrupted way, the electromagnetic field produced by the card during the frames. This is the second situation.
In this second situation, depending on the information transmitted, the electromagnetic field generated by the card during the frame may have the same phase or a phase opposed to that of the magnetic field generated by the reader.
Consequently, the amplitude of the generated magnetic field decreases in inverse proportion to the cube of the distance.
Thus, at the card antenna, the electromagnetic field emitted by the card is at a higher level than that which has been received by the card (generated by the reader) and on which synchronization must take place. For guidance, the ratio between the level of the electromagnetic field emitted by the card and that received by the card (generated by the reader) may be as much as 60 dB.
In the first situation (bursts separated by spaces during each transmission frame), synchronization may take place within these spaces between the carrier signal generated by the reader and the ALM carrier clock signal generated by the card.
However, this is currently impossible in the second situation mentioned above, owing to the card's continuous generation of its own electromagnetic field during each transmission frame.
In this case, therefore, synchronization currently takes place during the reception phase preceding each transmission frame. This synchronization therefore takes place outside the frames. However, this solution is unsatisfactory.
In fact, the phase difference between the reader carrier signal and the ALM clock signal increases during each frame and is proportional to the length of the frame.
Moreover, this synchronization outside the frame requires an external clock reference, which increases the power consumption.